How to get an URL for a resource directory when using OneJar?

Question

I need to pass the root "something" to a piece of code working with resources. I know that this "something" can't be a file, since this doesn't work in a JAR. So I'm passing an URL.

Before I started with OneJar I transformed an URL like

file:/some/path!/foo/bar/MyUrlSupplier.class

into

file:/some/path!/foo/images/

and used this as the root URL. With OneJar I'm getting

onejar:foo.bar.MyUrlSupplier.class

and transforming it into

onejar:foo.images

but whatever I do with this URL, nothing works. The best I get is this stacktrace caused by loading new ImageIcon(url):

Uncaught error fetching image:
java.lang.ClassCastException: sun.misc.Launcher$AppClassLoader cannot be cast to com.simontuffs.onejar.JarClassLoader
    at com.simontuffs.onejar.JarClassLoader.getByteStream(JarClassLoader.java:753)
    at com.simontuffs.onejar.Handler$1.getInputStream(Handler.java:50)
    at sun.awt.image.URLImageSource.getDecoder(URLImageSource.java:126)
    at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:263)
    at sun.awt.image.ImageFetcher.fetchloop(ImageFetcher.java:205)
    at sun.awt.image.ImageFetcher.run(ImageFetcher.java:169)